1. Field of the Invention
The present invention relates to compositions useful as coatings, particularly powder coatings which do not tend to cake or dust after being applied to the surface of a substrate, and coating compositions containing pigments which do not tend to discolor objects with which it comes into contact. The present invention is also directed to composite products of a substrate having a surface at least partially coated with such coatings, and to processes for the preparation of such coating compositions.
2. Discussion of Background and Material Information
The agricultural chemical industry encompasses a wide range of products originally developed for agricultural application, but currently expanded to diverse uses. For example, synthesized fertilizers were and remain an important agricultural product, being basic or essential chemical compounds for use by farmers in growing crops. Synthesized fertilizers often are composed of nitrogen, phosphorous and potassium-containing compounds. Although the primary purpose may be to deliver specific chemicals into the plant growing environment, it has been recognized that other chemicals, in smaller amounts (therefore termed "trace elements"), serve a useful purpose. Therefore, the synthetic fertilizer compositions will often contain these elements in addition to the primary chemicals identified above.
A synthesized fertilizer may be applied to the soil as well as directly to the plant itself. In the manufacture of such fertilizers, the relative ratios of these and other chemicals are controlled for particular applications. When fertilizers are applied to growing crops, either liquid or solid media forms can be used. Solid forms suitable for this purpose are typically either prills or granules. Although in either case, each form is approximately spherical granules are formed by an agglomerating process building layer upon layer whereas prills are formed by the solidification of a single droplet.
As the technology for producing such solid fertilizers has progressed, agricultural chemical manufacturers discovered that the coatings for prill or granule materials can be formulated from substances in which additional materials or additives, which may be referred to herein as "agricultural chemical additives", such as trace elements necessary for growing crops, biocides, herbicides, fungicides, nematicides, acaricides, bird repellants, growth regulators, soil structure improvement agents, pesticides, helpful biological species such as bacteria, or other such materials, are incorporated.
More recently, the chemical compounds normally used as fertilizers have been formulated to contain combinations of chemicals useful in chemical reactions other than those reactions which contribute to the growth of a plant. Thus, agricultural chemicals and derivatives from agricultural chemicals have been discovered to be particularly suitable for use beyond traditional farming applications, such as the use of ammonium nitrate as an explosive.
Such chemicals, however, often require special handling because of their reactive nature. Therefore, procedures involving the treatment and handling of chemicals and materials in manufacturing processes used to produce such combinations of chemicals was and remains an important area of technology, particularly for reasons of safety. The coating technology useful in the manufacture, handling , storage and application of agricultural chemicals has, therefore, become important for other industrial chemicals as well.
In the agricultural fertilizer industry, coatings on fertilizer solids have been used as anti-caking agents to prevent individual solids particles from caking together during the manufacturing, storing, handling and use. Such caking may result due at least in part to physical or chemical processes causing the individual particles to bond or stick to each other. Caking also tends to occur when moisture comes into contact with the particles.
Historically, clay has been the most commonly used anti-caking powder for coating fertilizer solids such as ammonium nitrate granules. Such clay coating may be applied by mixing fertilizer granules with clay particles so that the clay particles stick to the exterior surface of the fertilizer granule. However, the use of clay gives rise to undesirable clay dust suspension in the surrounding environment when the granules are handled, used or otherwise moved since such movement can cause dust to separate from the fertilizer granule. Also, if suitable clay is not close at hand to the manufacturing site, the cost of clay can become a factor due to the affect of its weight on shipping costs.
In recent years, commercial grade magnesium stearate powder has been employed as an alternative to clay for coating ammonium nitrate fertilizer granules. The designation of "commercial grade" indicates that the powder may have some impurities in the powder, at least more than might be found in a "pharmaceutical grade". The magnesium stearate coating has been used for this purpose in view of its anti-caking properties and reduced dust generation which are generally considered to be an improvement over clay.
The commercial grade magnesium stearate powder suitable for this purpose is normally made by a conventional "precipitation" or "fusion" processes. In precipitation, the materials to be reacted are dissolved in liquid and are brought together to react to form an undissolved solid which is recovered from the liquid. In fusion, melted materials are mixed together to react and the reaction product is then cooled to a solid. In either case, these processes involve reacting stearic acid with magnesium oxide. The commercial grade magnesium stearate powder produced by such reactions normally contains about 6-8% weight unreacted excess magnesium oxide (theoretically 6.8% MgO).
Although magnesium stearate powder is considered to be an improvement over clay as a fertilizer coating, commercial grade magnesium stearate powder has two undesirable characteristics which are pertinent to the present invention. Firstly, magnesium stearate powder is relatively expensive and thus contributes substantially to the cost of the fertilizer. Secondly, magnesium stearate powder is white in color. Consequently a composite fertilizer product, for example white ammonium nitrate granules coated with white commercial grade magnesium stearate, is indistinguishable from other white fertilizer, such as urea. This lack of distinguishing color is undesirable and potentially hazardous because it can lead to inadvertent mixing of, for example, the ammonium nitrate granules with a second fertilizer which can result in adhesion or "mudding" of the mix which would become extremely hydroscopic in nature and result in mushing because of the retained moisture.
In a prior attempt to color commercial grade magnesium stearate powder coating material, the assignee of the present invention physically mixed dry green pigment powder with magnesium stearate powder. However, there were three significant problems associated with that effort. Firstly, the distribution of the coloring agent pigment was variable, and it was difficult if not practically impossible to effect an homogenous distribution of pigment to result in a uniform color. Secondly, a relatively large concentration of pigment was required, which affected the cost of the end product. Thirdly, and most significantly, when the coating material was applied to a substrate the pigment had a tendency to rub off easily and, thus, discolored anything with which it came into contact.
U.S. Pat. No. 3,966,853 and U.S. Pat. No. 4,736,683, relate to the use of magnesium oxide to improve the physical prill properties, such as hardness and resistance to caking of ammonium nitrate prills, such as ANFO explosives. In those patents, magnesium stearate, calcium stearate and other metallic carboxylic acid salts are mixed with particulate ammonium nitrate in drums to improve physical properties.
U.S. Pat. No. 4,036,979 is directed to wherein the use of magnesium stearate and zinc stearate as a lubricant wherein a method is disclosed for improving feed efficiency and accelerating animal growth, the lubricating property being used on an implant.
U.S. Pat. No. 4,605,550 relates to controlled or delayed release formulations in which magnesium stearate is used as a lubricant for the die wall.
In the field of agricultural and industrial chemicals, however, there exists a need to provide a coating for substrates, such as chemical particles, which is relatively inexpensive, is improved with respect to reducing dust generation, experiences minimal caking, and which can be tinted using a color other than white or the base color of the substrate being coated, for example so that the coating will distinguish the coated chemical particle from other granular fertilizers or chemicals.
Notwithstanding attempts to provide an improved coating for agricultural and industrial chemicals, and specifically for improved magnesium stearate coatings, prior to the present invention, there was still a need for a magnesium stearate coating wherein the coating material would not cake or dust and particularly such a magnesium stearate coating which included a pigment which had a reduced tendency to transfer color to an object which comes into contact with the pigmented coating composition or a substrate coated with such a pigmented coating composition.
The present invention is believed to overcome the problems and disadvantages discussed above by providing an economical and efficient coating composition for substrates which prevents caking and minimizes dusting and permits coloration of the coating composition without subsequent discoloration of objects which come into contact with the coating composition.